This invention relates to a process for producing a 2-pentenoic ester which comprises isomerizing the corresponding 3-pentenoic ester.
In general, many prior processes for isomerizing unsaturated carboxylates are known. For example, (a) J. Chem. Soc. 2454 (1957) discloses a process for isomerizing a 3-hexenoic ester in the presence of potassium hydroxide as a catalyst; (b) J. Org. Chem. 35 3352 (1970) discloses a process for isomerizing methyl 3-pentenoate in the presence of sodium methylate or iron pentacarbonyl; and (c) Bull. Chem. Soc. Jpn., 51 2970 (1978) discloses a process for isomerizing dimethyl methylenesuccinate to dimethyl 2-methylbutenoate in the presence of triethylamine.
However, in process (a), the esters employed as a raw material and isomerized are hydrolyzed by the alkali hydroxide to form the alkali salt of unsaturated carboxylic acids. In order to avoid this shortcoming, the alkali salt of unsaturated carboxylic acid should be neutrilized and esterified. This is complicated.
In process (b) in which sodium methylate is used, sodium methylate adds to the double bond of the unsaturated carboxylate. This lowers the yield of 2-pentenoic ester. In process (b) in which iron pentacarbonyl is used, the reaction speed is slow and, also, the reaction is necessarily effected under an atmosphere pressurized by carbon monoxide, because iron pentacarbonyl is unstable.
In process (c), isomerization of dimethyl methylenesuccinate to dimethyl 2-methylbutenoate proceeds in the presence of triethylamine even at room temperature, and little side reaction occurs. However, isomerization reaction of a 3-pentenoic ester to the corresponding 2-pentenoic ester in the presence of triethylamine slowly proceeds even at 100.degree. C. This is too slow to be industrially practicable.
In addition, these known processes are effected in a liquid phase homogeneous reaction. In this case, an apparatus for separating the catalysts from the resulting reaction mixtures is necessary. This makes the process unsatisfactory from an industrial point of view.